I. Field of the Invention
The invention is directed to an improved method of recovering viscous oil from a subterranean oil formation wherein it is difficult to establish thermal communication between the injection and the production wells.
II. Description of the Prior Art
Many oil reservoirs, such as heavy oil or tar sand formations, exist which contain vast quantities of oil not amenable to recovery by conventional methods because the oil is so viscous that it is substantially immobilized at reservoir conditions. Therefore, some form of supplemental oil recovery must be used in such formations to sufficiently decrease the viscosity of the oil to allow it to flow through the formation to the production well and then to the surface of the earth. Thermal recovery methods decrease the viscosity of such oil and are therefore suitable for stimulating the recovery thereof.
Steam has been utilized in the past for thermal stimulation of viscous oil in steam drive or steam flooding processes in which steam is injected into the formation on a substantially continuous basis through an injection well, and the oil, having reduced viscosity, is recovered from the formation through a spaced-apart production well. The mechanism of the oil production by steam flooding is believed to involve the condensation of the steam upon contact with cooler formation fluids including the viscous oil, thereby reducing the viscosity of the oil and allowing it to flow more easily. The establishment of thermal communication between the injection and the production wells in such steam-flooding processes is essential for achieving increased oil recovery. In most formations subjected to steam flooding, the injection of steam through the injection well combined with steam stimulation of production wells produces sufficient thermal communication between the injection and the production wells within a reasonable time period to provide a sufficiently increased production of oil from the formation.
The injection and the production wells are normally arranged in patterns where well spacing is small, ranging from about 1.25 to about 5 acres, depending upon formation thickness and steam generation capacity.
While it is relatively easy to establish thermal communication between the injection well and production wells which are subjected to the action of steam from more than one injection well, it is relatively difficult to establish thermal communication with at least some wells that are subjected to the action of steam from only one direction, i.e., outer-ring wells. Unless thermal communication is established between the injection well and a cold, e.g., less than 200.degree. F., production well, the production of oil from such wells is not enhanced by steam flooding and, therefore, steam flooding is not utilized to its fullest potential.
Accordingly, it is a primary object of the present invention to provide an improved method of recovering viscous oil by steam flooding of a relatively heavy oil-containing reservoir.
It is an additional object of the invention to provide an improved method of providing thermal communication and thermal breakthrough between the injection well and all of the production wells in the formation.
These and other objects of the invention will become apparent to those skilled in the art from the following description thereof.